Substances, petrophysical properties, and geodynamic conditions for the formation of granitoids in the Dakhovsky crystalline massif (Western Caucasus)

Abstract

Introduction: The article provides the results of a comprehensive research into the mineral and chemical composition, geochemistry, and petrophysical features of granitoids in the Dakhovsky crystalline massif (DCM). Methodology: Analytical studies of the material composition included the production and description of microsections, X-ray fluorescence and X-ray diffraction analyses. The results of X-ray fluorescence analysis were reduced to norms using the Petro Explorer V.3.2.0.2 program. Heavy concentrates from the grinding samples of the main types of rocks were studied to characterise the accessory minerals. The geophysical characteristics of the granitoids were determined by selected samples and in the outcrops. They included density, magnetic susceptibility, longitudinal wave velocity, and electrical resistivity. Results and discussions: The results of the fluorescence analysis revealed a wide range of granitoids depending on interstitial phases. First phase rocks vary from quartz diorites and quartz monzodiorites to gran-odiorites and granosyenites. Second phase rocks correspond in composition to granites and leucogranites of moderate alkalinity. X-ray diffraction analyses showed the presence of tridymite in granitoids, which allowed adjusting the calculated melting temperature of the primary melt. Also, it revealed two varieties of chlorites and a broad range of metal compounds. Melting temperatures above 1,000° are characteristic of diorites, and below 1,000° of granodiorites of the first interstitial phase. The calculated temperature for the second phase granites ranges between 765° and 895°. Conclusions: The results of the studies, including the variability from diopside- to corundum standard, as well as the range of accessory minerals from granitoids of both phases, such as orthite and magnetite, indicate the subcrustal origin of the parental melts with partial influence of the core substrate responsible for the formation of granitoids in the DCM. Petrophysical characteristics of granitoids of various types were defined and their interconnections and connections with the composition characteristics, in particular, the content of various oxides, were studied. The obtained petrophysical characteristics in combination with other composition data can be regarded as reference characteristics in the regional research and generalisation of the material on magmatism in Northern Caucasus.